In most processes for packaging products, such as potato chips, nuts, candy, biscuits, spices and similar foods, hardware such as nuts, bolts, screws and nails, the package is formed and filled by creating a heat-seal between two opposed layers of film to form a pocket and almost simultaneously sliding or dropping the product into the pocket. In these form and fill packaging techniques a continuous flat web of packaging film is fed around a form which shapes it into a tube, the tube is slipped over a hollow form and the free edges of the tube are sealed together. The tube so formed is then passed between a pair of hot sealing jaws which create a series of discrete packages by collapsing the film onto itself and forming a seal by the application of heat and pressure. The product is introduced into each package through the hollow form in the interval between the heat seals. During high operating speeds, the product is dropped into the package while the sealing jaws, which form the seal, are closed. With both vertical and horizontal form and fill sealing applications the heat seal should be strong enough to support and retain the product after the sealing jaws open to release the film. It is often desirable to release the sealing jaws soon after the seal is formed so a film which accomplishes this by exhibiting a high "hot tack" is very useful.
Additionally, in packaging applications there is a great demand for heat sealable films which can be subjected to temperatures high enough to seal the films without causing the substrate to cockle or pucker. One approach for achieving this is by coating a film substrate with a layer of heat sealable material which adheres strongly to the substrate and which can be melted at a temperature below the softening temperature of the substrate. Heat-sealable coatings with low melting temperatures are often preferred because the substrate is less likely to be damaged during heat sealing.
In the preparation of films useful for packaging purposes, the outside of the film or the side of the film which comes in direct contact with the hot sealer surfaces should have good hot slip and jaw release characteristics. Additionally, the film should have good machinability so that the wrapped product can be conveyed easily through the overwrapping machine without sticking to adjacent packages or the parts of the machine with which it comes into contact, which can cause production delays. The film should also have barrier properties. Acrylic-containing coatings which offer these properties are known. The acrylic-containing coating is applied to one side of the film substrate and another heat sealable coating, such as polyvinylidene chloride (PVDC), or another acrylic coating, is coated on the other side. Acrylic-containing coating formulations provide the film with a good coefficient of friction which contributes to good machinability characteristics. These acrylic-based coatings also provide the film with good barrier characteristics which improve flavor and aroma protection. Such coatings are described in U.S. Pat. Nos. 4,058,649 and 4,058,645. The PVDC coating or other type of acrylic coating is usually on the inside of the film and provides high seal strength, good hot tack characteristics and barrier properties. These heat sealable coatings have glass transition ("Tg") temperatures which are higher than room temperature. Such a coated film is disclosed in U.S. Pat. No. 4,403,464.
In the film packaging art, materials which can contribute to film damage by causing the film to pucker are often found either in the packaging materials themselves or in the packaged products. It has been found that oriented polypropylene films having acrylic and ethylene-acrylic acid copolymer-containing coatings are sensitive to some types of food additives, for example, the proprietary release agents used in starch based candies. These coatings can also be sensitive to natural oils found in spices such as black pepper, anise and nutmeg. When placed in contact with these aggressive materials, the oriented polypropylene films pucker and deform giving the package a wrinkled and pocked appearance. This can happen within hours of exposure at temperatures above about 29.degree. C. (85.degree. F.) and high humidity (above about 80% relative humidity) or even within two weeks at ambient temperatures and ambient humidity.
One solution to this problem has been to use a polyester film coated with PVDC. The PVDC coating offers moisture barrier properties. These films are useful in packaging starch-based candies, to preserve softness and prevent hardening. The polyester is not affected by the reactive materials used in the products. Significant amounts of PVDC are required for effective barrier properties. However, PVDC contains chlorine, a halogen Halogenated substances have been the subject of environmental concern, and for this reason it is desirable to minimize the use of PVDC in films. Furthermore, polyester films usually have to be laminated to another web, such as polyethylene, for structural integrity (stiffness). This is undesirable because it is an expensive process step and the packaging poses recycling problems because of the two-web structure. Moreover, the polyester films are more expensive than oriented polypropylene.
U.S. Pat. No. 4,456,741 discloses heat sealable terpolymer compositions useful as pressure-sensitive adhesives for use with a backing material such as paper, polyester film or foamed polymers. The terpolymer heat sealable pressure-sensitive adhesive composition comprises butyl acrylate, N-vinyl-2-pyrrolidinone and styrene. Other heat sealable coatings are disclosed in U.S. Pat. No. 3,696,082; and East German Patent No. DD-146,604.
Cold sealable pressure-sensitive adhesives have been developed. These adhesives do not require the use of a heated element to seal the packages. However, these adhesives have high surface tack characteristics making them adhere to uncoated surfaces of the packaging film which makes them difficult to use because of film blocking (i.e. sticking).
Although low blocking coating compositions with a low seal temperature (of about 71.degree. C. (160.degree. F.)) can be obtained with ethylene-acrylic acid copolymer coatings, such as Mobil Chemical Company film product "100 LTSC", these polymeric coatings do not have adequate machinability for use on the outside of the web and will not seal to other types of acrylic-containing coatings. Therefore, acrylic or PVDC coatings (or other machinable surface coating) must be used on the outside of the packaging film. This limits such films to "fin-seal" type applications in which the inside of the film is sealed to itself. Many applications, however, require a film structure which can be lap sealed, in which the inside surface is sealed to the outside surface.
It is apparent from this film art that there is a need for a sealable coating formulation which has the good flavor and aroma properties of acrylic and the low temperature sealability of the ethylene acrylic acid but which is not sensitive to reactive materials and which does not stick to acrylic coatings at room temperature or temperatures below about 60.degree. C., but which seals to acrylic coatings at or above 70.degree. C. Additionally, a film is needed which is resistant to reactive materials without containing halogenated substances and which is easy to recycle.
Sometimes the advantages of adding to the substrate materials which offer moisture and aroma barrier properties will outweigh the disadvantages which compromise the appearance and protective properties of the film and cause handling problems. Some additives incorporated into the film substrate to improve the moisture and aroma barrier properties of the film will migrate from the substrate to the surface of the film and cause various problems, depending upon the additive. Terpenes are useful moisture barrier additives and for this purpose they are often incorporated into oriented polypropylene film substrates. However, terpenes have been found to migrate through the known coatings used on oriented polypropylene substrates. This additive migration has been found to reduce the hot tack of the film, which is particularly important in the vertical form and fill sealing processes. Terpene migration has also been reported to cause flavor degradation of foods. The components of some films will impart an undesirable odor and/or flavor to foods and other products.
Thus, there is a need for a coating composition for polypropylene which is not damaged by these aggressive materials and which can block migration of terpenes and other hot melt polymers.